Migration executor cannot handle AlterField operation on ForeignObject

[Jacob Walls]

Starting with the four models in the composite primary key docs, altering the model with ForeignObject to become a ForeignKey (while also changing the related model to no longer use a composite primary key), produces a migration that cannot run. Failure is similar to #35992 and #35997.

To reproduce:

• Copy the four models from the ​docs
• makemigrations
• Use new models:

class Product(models.Model):
    name = models.CharField(max_length=100)


class Order(models.Model):
    reference = models.CharField(max_length=20, primary_key=True)


class OrderLineItem(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    order = models.ForeignKey(Order, on_delete=models.CASCADE, primary_key=True)
    quantity = models.IntegerField()


class Foo(models.Model):
    item_order_id = models.IntegerField()
    item_product_id = models.CharField(max_length=20)
    item = models.ForeignKey(
        OrderLineItem,
        on_delete=models.CASCADE,
    )

• makemigrations
• migrate

Running migrations:
  Applying models.12009_order_product_orderlineitem_foo... OK
  Applying models.12010_remove_orderlineitem_pk_alter_foo_item_and_more...Traceback (most recent call last):
  File "/Users/<user>/prj/arches/manage.py", line 27, in <module>
    execute_from_command_line(sys.argv)
    ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/Users/<user>/django/django/core/management/__init__.py", line 442, in execute_from_command_line
    utility.execute()
    ~~~~~~~~~~~~~~~^^
  File "/Users/<user>/django/django/core/management/__init__.py", line 436, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/Users/<user>/django/django/core/management/base.py", line 416, in run_from_argv
    self.execute(*args, **cmd_options)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/<user>/django/django/core/management/base.py", line 460, in execute
    output = self.handle(*args, **options)
  File "/Users/<user>/django/django/core/management/base.py", line 107, in wrapper
    res = handle_func(*args, **kwargs)
  File "/Users/<user>/django/django/core/management/commands/migrate.py", line 353, in handle
    post_migrate_state = executor.migrate(
        targets,
    ...<3 lines>...
        fake_initial=fake_initial,
    )
  File "/Users/<user>/django/django/db/migrations/executor.py", line 135, in migrate
    state = self._migrate_all_forwards(
        state, plan, full_plan, fake=fake, fake_initial=fake_initial
    )
  File "/Users/<user>/django/django/db/migrations/executor.py", line 167, in _migrate_all_forwards
    state = self.apply_migration(
        state, migration, fake=fake, fake_initial=fake_initial
    )
  File "/Users/<user>/django/django/db/migrations/executor.py", line 255, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/Users/<user>/django/django/db/migrations/migration.py", line 132, in apply
    operation.database_forwards(
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        self.app_label, schema_editor, old_state, project_state
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/<user>/django/django/db/migrations/operations/fields.py", line 236, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
    ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/<user>/django/django/db/backends/base/schema.py", line 848, in alter_field
    if not self._field_should_be_altered(old_field, new_field):
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/<user>/django/django/db/backends/base/schema.py", line 1690, in _field_should_be_altered
    return self.quote_name(old_field.column) != self.quote_name(
           ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/Users/<user>/django/django/db/backends/base/schema.py", line 207, in quote_name
    return self.connection.ops.quote_name(name)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/<user>/django/django/db/backends/postgresql/operations.py", line 197, in quote_name
    if name.startswith('"') and name.endswith('"'):
       ^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'startswith'

[Jacob Walls]

Setting back to Normal, realized we have this disclaimer:

Django doesn’t support migrating to, or from, a composite primary key after the table is created.

Perhaps this is an "optimization" ticket then.

[Jacob Walls]

Just chiming in to add that, as I mentioned, this is reproducible without "migrating from a composite primary key" but rather migrating away from the ForeignObject. E.g., making this single change from

    item = models.ForeignObject(
        OrderLineItem,
        on_delete=models.CASCADE,
        from_fields=("item_order_id", "item_product_id"),
        to_fields=("order_id", "product_id"),
    )

to

    item = models.IntegerField()

reproduces the issue.

[Sarah Boyce]

Thank you! Feels a little similar to #34820